It is estimated that about 25% of the world crop production is lost due to microbial spoilage, of which spoilage by fungi is by far the most important cause. Not only from an economical point of view, but also from a humane point of view it is of great importance to prevent spoilage of food products. After all, in many parts of the world people suffer from hunger.
In that respect bananas are an important crop. Bananas are ranked fourth after rice, wheat and maize in human consumption. They are a valuable source of vitamin B6, vitamin C and potassium. The banana plant is an herb belonging to the genus Musa and is grown in more than hundred countries worldwide. Although being cultivated primarily for their fruit, they are to a lesser extent also used for the production of fiber and as ornamental plants.
Fusarium wilt, also called Panama disease, is caused by Fusarium oxysporum. It is the most widely spread disease of banana plants and, historically, the most important disease of bananas. Well-known epidemics devastated based export plantations up to the mid-1900s, and locally consumed cultivars continue to be affected worldwide. The introduction of resistant Cavendish cultivars in the tropics saved the international banana export trade industry during the 1960s. During the 1970's, however, Cavendish bananas succumbed to the disease in subtropical countries such as South Africa.
The disease has spread through plantations in Australia, South Africa and parts of Asia. It is only a matter of time before it reaches the hub of commercial production in Latin America and the Caribbean. No control strategy has been found to be effective in combating the disease, and most success has been achieved by preventative measures such as the isolation of infected plants and the planting of tissue culture plants in disease-free fields. Today, however, Panama disease is again threatening the world banana production.
Next to Fusarium oxysporum, the moulds Mycosphaerella fijiensis and Mycosphaerella musicola also cause diseases of banana leaves, the so-called Black and Yellow Sigatoka disease.
The production of bananas is most endangered by the Sigatoka leaf spot or the ‘black leaf streak”, caused by Mycosphaerella fijiensis. Black Sigatoka occurred for the first time about thirty years ago in some Central American countries. Within 10 years, Black Sigatoka became the predominant leaf disease in bananas with a self-accelerating spread to all important banana growing areas in Central and South America, Central and West Africa and wide areas of Asia.
The less dangerous Yellow Sigatoka disease occurs in banana plantations for several decades now. The disease attacks leaves during the entire growth period up to the time of harvesting. It initially causes spotting and blotching of the leaf surface that results in necrosis and withering of the leaf tissue. The course of the disease is usually slow, but the reduction of the active leaf surface leads to a weakening of the plant and an associated loss in yield. Because of its slow development, it was possible to control the disease by spraying the plants with mineral oil or with a mixture of mineral oil and fungicides.
Black Sigatoka differs from Yellow Sigatoka in its much more aggressive occurrence and in a disease cycle twice as rapid. The young leaves are infected even during formation and fade within 4-5 weeks. In addition to attacking all worldwide important banana table varieties, the disease also attacks the plantain fruit that constitutes the diet of the native population in wide areas of the tropical belt. Black Sigatoka has completely displaced Yellow Sigatoka in the most important banana growing areas.
The aggressive and epidemic occurrence of Black Sigatoka, especially in the tropical growing regions of America, Africa and Asia with their high rainfalls, leads to a rapid destruction of the banana plants. Infected leaves blacken, become necrotic and disintegrate. Planned production of bananas without appropriate protection against Black Sigatoka is no longer possible.
Spraying banana plants with chemicals is currently applied. The benzimidazole fungicides introduced at the beginning of the 1980's were very effective when sprayed at intervals of 2-3 weeks. However, due to the mechanism of action of this class of products and to their frequent application, Black Sigatoka developed an almost complete resistance to the benzimidazole deverivates within a few years.
For some years, triazole fungicides have been used with good success against Black Sigatoka. However, the number of spray applications per year had to be severely limited since their introduction on the market, in order to prevent the development of resistance. Despite this measure, a huge decrease in sensitivity has already been observed.
Further treatments have been suggested in e.g. WO 97/47202, wherein a fungicidal composition comprising natamycin and a fungal cell wall degrading enzyme and its use in the field of crop protection is described. In WO 2005/074687 an antifungal composition containing natamycin to prevent growth of pathogenic moulds on banana plants is described. In U.S. Pat. No. 4,148,891 an anti-fungal composition comprising natamycin, a lower alkanol and a lower alkanoic acid for the protection of agricultural products from mould growth is described.
It has been recognized that the world's most popular fruit and a basic staple food for hundreds of millions of people in the developing world—the banana—is under severe threat. Mycosphaerella fijiensis and Fusarium oxysporum f. sp. cubense can cause the extinction of the banana within 10 years. This would be a disaster to the 500 million Africans and Asians that are dependent on the production of bananas.
Producers, who can afford pesticides, spray the cultures up to 50 times a year. This is equivalent to ten times the average frequency applied in intensive agriculture plants of industrialized countries. The sprayings are not only expensive, making up a quarter of production costs, but present a serious risk to workers and a threat to the environment.
Next to the intensive use of pesticides, a much less aggressive method of control is the improvement of cultural methods. An early warning system has been developed to control Black Sigatoka. The system is based on weekly observations of symptoms on leaves of young plants followed by target fungicide applications within specific periods when disease severity is starting to increase and environmental conditions are favourable for disease development.
Cultural methods play an important role in reducing conditions for development of the disease. But, despite these control measures, the survival of edible banana species are seriously threatened by the Sigatoka and Panama diseases. No effective methods of preventing growth of fungi on banana plants, especially the growth of Mycosphaerella fijiensis and Fusarium oxysporum f. sp. cubense, without risk of development of resistance and without danger for the health of exposed persons and the environment, are presently known.
A further major plant disease problem for which conventional fungicide control is both intensive and a burden for the environment is early blight disease of potato. Potatoes are plants of the Solanaceae family. The potato is traditionally strongly associated with Europe, the United States of America and Russia because of its large role in the agricultural economy and history of these regions. But in recent decades, the greatest expansion of potato has been in Asia, where as of 2007 approximately eighty percent of the world potato crop is grown. Since the dissolution of the Soviet Union, China has become the world's largest potato producer, followed by India.
Potatoes are generally grown from the eyes of another potato and not from seed. Some commercial potato varieties even do not produce seeds at all and are propagated only from tuber pieces. These tubers or tuber pieces are called “seed potatoes”.
Early blight disease is caused by the fungi Alternaria solani and Alternaria alternate. The disease is the major potato disease in the USA and constitutes an increasing problem in Europe. Alternaria produces small darkened lesions on the potato plants that spread into growing black spots of dead tissue, often killing most of the plant in the long run. Seeds infected with the disease may even damp off during germination. This disease can be prevented with some fungicides, including azoxystrobin. However, intensive use of fungicides has caused a widespread shift in Alternaria population towards species and strains that are resistant to the most commonly used products. The danger for the health of exposed persons and the environment is a further serious disadvantage of the currently used products.
Consequently, it can be concluded that there is a severe need for more effective, more environmental friendly, lower-toxicity and less harmful antimicrobial compounds/compositions, e.g. antifungal compounds/compositions, for the treatment of mould growth in and on banana and potato plants.